The invention relates to lead-acid batteries having improved performance compared with current lead-acid batteries.
Batteries of this type are particularly intended for the production of high-performance batteries, for electric vehicles for example.
During the 1970""s, research into high performance batteries intended for electrical vehicles confirmed a known dilemma, but which particularly concerns lead-acid batteries: choosing between weight performance and endurance, improvement in one being achieved to the detriment of the other. At times priority was given to energy-to-weight ratio to such an extent that lifetimes fell to scarcely acceptable values. Therefore research was conducted to arrive at a better compromise.
During a 5-hour discharge under constant current, the energy-to-weight ratio of lead-acid batteries intended for electric vehicles ranges from 30 to 40 Wh/kg. On board vehicles, this range of magnitude is reduced by 20%. Therefore, at discharge times of 1 to 2 hours, the lead-acid battery proves to give two times less performance than the nickel-cadmium battery, and three times less than the sodium-sulphur battery.
It would therefore appear that the lead-acid battery ranks the lowest among possible candidates for the market of electric vehicles. Yet, over and above its price, some prospects plead in its favour.
Its performance in terms of charge time should increase with the arrival of specific vehicles that are lighter and more energy-saving. Its lifetime could be increased through intelligent management of its energy. Moreover, the lead-acid battery is probably, among those batteries competing for the new markets, the one which has the highest relative margin for progress.
To improve the performance of lead-acid batteries, it is required to increase the coefficient of use of the active materials of the electrodes. Two pathways of research can be considered to achieve this result. One concerns the collection of charges on the side of the active electrode materials. The other concerns a better distribution of the reagents within the electrodes.
The active material of the electrodes is in fact only used very little, even during a so-called complete discharge, the percentage of converted material being in the region of 25 to 30%.
In respect of charge collection, the electron exchanges between the active material and the outer circuit are ensured by lead alloy conductors in the form of grids or ribs. The size of these collectors is determined by the two following factors: the fact that they form the mechanical support for the electrodes, and the need to resist corrosion phenomena to which they are subject inside the electrodes. In consequence, in current assembly technology, so-called single pole assembly, the weight of the collectors represents between 40 and 50% of the weight of the electrodes for electric vehicle applications.
Regarding the distribution of reagents inside the electrodes, this is limited by the electrode porosities which may be used, as will be seen below.
For over a century, the plates of lead-acid batteries have been made using the Faurxc3xa9 method, the so-called grid and added oxide method. More precisely, a grid in lead alloy is lined with a paste made of lead oxide, sulphuric acid and water. The proportions of these various constituents were empirically determined having regard to the performance of the battery. It soon became apparent that the increase in the quantity of water, that is to say in resulting porosity, increases initial capacity to the detriment of lifetime. Values held to be acceptable for these two parameters limit variations in porosity to a narrow range, in the region of 10%.
Inside an electrode of a lead-acid battery, the quantity of sulphuric acid is much lower than the quantity of active matter likely to be oxidized or reduced. Their ratio is in the order of 15%. To this acid, present on site, additions are made during discharge brought by diffusion from outside the plates, which are more significant the slower the discharge. The coefficients of use of the active matter are highest in the vicinity of the surface of the plates. They may reach 60 to 70%, to be compared with the average value of 25 to 30% for the entire electrode.
Document FR-A-2 438 346 [I] and the publication by J. Alzieu et al., at the Fifth International Electric Vehicle Symposium, Philadelphia, Oct. 2-5, 1978 [2], describe lead-acid batteries with a long lifetime. These batteries have a positive electrode, a negative electrode, an electrolyte formed of sulphuric acid, a set of separator elements arranged between the positive electrode and the negative electrode and means for applying pressure to the whole assembly. It is indicated that with the application of pressure it is possible in particular to increase the lifetime of these lead-acid batteries.
The document J. Electrochem. Soc., vol 130, No. 11, 1983, pages 2144-2149 [3] illustrates a lead-acid battery which uses an active material for the positive electrode having a density of 3.9 g/cm31, and an electrolyte made of sulphuric acid having a density of 1.28 at 20xc2x0 C. With the use of such materials it is possible to restrict changes in the structure and physical properties of the active material of the positive electrode, for the purpose of improving its lifetime. In this document, pressure is also applied to the assembly of electrodes, by which means it is also possible to limit electrode structural changes.
The improvement brought by placing the electrodes under stress is of interest, but the energy-to-weight ratio of lead-acid batteries still remains insufficient compared with the performance it is desired to obtain.
Other improvements in lead-acid batteries have been considered by H. Ozgun et al., Journal of Power Sources, 52, 1994, pages 159-171 [4]. These improvements concern variation in the density of the active material of the electrodes. In this latter case, the authors recommend increasing the density of the active material in order to increase the battery""s charging/discharging cycle capacity.
P. W. Appel and D. B. Edwards in Journal of Power Sources, 55, 1995, pages 81-85 [5] endeavoured to improve the performance of the lead-acid battery by improving the conductivity of the active material through incorporation of conductor particles, but they did not succeed in finding particles that were able to withstand the particularly corrosive medium of a positive electrode in a lead-acid battery.
Application of pressure to the electrode should bring about an improvement in the coefficient of use of the reagents inside the electrodes by using electrodes in the form of thin plates. It can be understood that a thin plate, that is to say in which every active material element is near a surface delimiting the plate, can have improved performance; the overall coefficient of use should be expected to be 60 to 70%.
With compression it is possible to remedy the special fragile nature of these thin plates. After considering a move in this direction, currently one of the lines of research adopted by the ALABA Advanced Lead Acid Battery Consortium, this approach has been abandoned since unsuspected experimental results have opened up new prospects.
The subject of the present invention is precisely a lead-acid battery of the type described in documents [1] to [3], which has an improved energy-to-weight ratio due to arrangements allowing an increase in the coefficient of use of the active materials of the electrodes by achieving a better distribution of the reagents within the electrodes.
The subject of the present invention is a leadacid battery containing:
a positive electrode containing lead oxide as active material,
a negative electrode containing lead sponge as active material,
an electrolyte formed of a solution of sulphuric acid,
a separator element between the positive electrode and the negative electrode, and
means for applying a stress to the entire assembly perpendicular to the plane of the electrodes, in which, in the charged state, the quantity of sulphuric acid in the positive electrode represents at least 0.20 mole of H2SO4 per mole of active material of the positive electrode, and/or the quantity of sulphuric acid in the negative electrode represents at least 0.20 mole of H2SO4 per mole of active material in the negative electrode.
This quantity of sulphuric acid in the positive or negative electrode may, for example, represent from 0.20 to 1 mole, or from 0.20 to 0.70 mole of H2SO4 per mole of active material in the electrode.
Preferably, according to the invention, in the battery in the charged state, the quantity of sulphuric acid in the positive electrode represents at least 0.25 and even better 0.40 mole of H2SO4 per mole of active material in the positive electrode, and/or the quantity of sulphuric acid in the negative electrode represents at least 0.25 and even better 0.40 mole of H2SO4 per mole of active material in the negative electrode.
In the lead-acid battery of the invention, this improved distribution of reagents within the electrodes can be obtained by adopting one or more of the following arrangements, compared with the prior art:
1) modifying the structure of the positive electrode in order to increase the quantity of sulphuric acid in the positive electrode,
2) modifying the structure of the negative electrode in order to increase the quantity of sulphuric acid in the negative electrode, and
3) increasing the H2SO4 concentration of the electrolyte.
According to the invention, it is possible to use simultaneously two or three of these modifications to adjust sulphuric acid quantities in the positive electrode and/or in the negative electrode to desired values.
For, according to the invention, it has been discovered that the qualities of sturdiness of lead-acid batteries achieved by electrode compression, by applying a compression stress to the electrodes in the order of 0.01 to 0.3 MPa, made it possible to apply other arrangements able to improve the coefficient of use of the active material in the positive electrode, whereas such arrangements would have been hazardous in a conventional battery structure.
These arrangements particularly concern:
increasing the H2SO4 concentration of the electrolyte,
increasing the porosity of the electrodes, and
including porous elements in these electrodes.
Therefore, according to a first embodiment of the invention, the porosity of the positive and/or of the negative electrode is modified. In this case, an electrode active material is used having an apparent density in the dry, charged state of 2.8 to 3.2 g/cm3, preferably from 3.0 to 3.2 g/cm3.
By increasing the porosity of the active material of the positive electrode and/or of the negative electrode, it is possible to increase the quantity of sulphuric acid per mole of active material, and therefore to promote exchanges between the active material of the electrode and the electrolyte.
To obtain an electrode active material with increased porosity, it is possible to proceed in the following manner.
As a general rule, the active material of the electrodes is obtained from a paste made of lead oxide, water and sulphuric acid by pasting a grid which serves to collect the current, followed by drying, then by maturing for 48 hours in a saturated steam atmosphere. By subsequently applying an electric current to the electrodes, lead dioxide PbO2 is formed which serves as active material for the positive electrode and spongelike metallic lead is formed which serves as the active material for the negative electrode. In this production, the water and acid content of the lead oxide paste regulates the porosity of the active material subsequently obtained. According to the invention, all that is needed therefore is to adjust the water and acid content of the paste to obtain an active material having an apparent density lying within the range described above.
A further means of increasing the porosity of the active material of an electrode of the prior art, is to submit this electrode which contains a material generally having an apparent density in the dry, charged state of 3.3 to 3.6 g/cm3, to electric treatment consisting of at least one deep discharge followed by recharging.
This may be carried out by placing an element, made of conventional electrodes, in short-circuit at the end of discharge, for 48 hours for example. The effect of this deep discharge is to swell up the positive electrode and/or negative electrode, and hence to increase its porosity. However, after 150 to 200 charging/discharging cycles under normal operating conditions, the porosity of the electrode may have decreased and returned to its initial value. In this case, the increased porosity value can be restored by causing the electrode to re-undergo at least one deep discharging cycle.
According to a second embodiment of the invention, the structure of the positive electrode and/or of the negative electrode is modified by adding inert porous particles, able to charge themselves with electrolyte, in the active material of these electrodes. These porous particles may be microporous fragments of inert material such as polyethylene, polypropylene or any other polymer resistant to the electrolyte. The presence of electrolyte in these porous particles also makes it possible to increase the quantity of electrolyte per mole of active material in the electrode. These porous particles may be added to the lead oxide paste used to line the grids or current collectors.
It is possible, in particular, to add a quantity of porous particles in the active material in such manner that they represent 5 to 80%, preferably 10 to 50%, of the final volume of the electrode material. In this way, the structure of the electrode active material is modified by increasing its porosity and its electrolyte content.
According to a third embodiment of the invention, the quantity of electrolyte is adjusted per mole of active material of the electrodes by increasing the density of the electrolyte. In this case, it is possible to use a solution of H2SO4 having a density of at least 1.30, for example from 1.30 to 1.50, and preferably from 1.32 to 1.40.
Advantageously, this third embodiment of the invention is combined with one of the two preceding embodiments.
Other characteristics and advantages of the invention will be better understood on reading the following description of examples of embodiment, which are evidently given for illustration purposes only and are not restrictive, with reference to the appended drawings.